The objective of this project is to develop the technology for identifying and visualizing active gene regions on animal cell metaphase chromosomes. This method is based on the sensitivity of active genes for the enzyme DNase I. Chromosome preparations will be nick translated in situ using DNase I and DNA polymerase and the substituted active regions visualized by autoradiography or modified nucleotide specific stains. Once operative it will then be possible to map on each chromosome all active gene clusters in any animal genome. The technique will also be applied to study biologically relevant questions at the molecular biological level. This technology will be especially useful for following the process of X inactivation and reactivation in female cells during embryonic development. It will also be used to characterize the localization of tissue specific genes and their mode of appearance during development. A chromosome map of active regions should provide important information about the overall structure of chromosomes and the relationship of active genes to other morphological markers which are detected by standard staining techniques. Finally this method can be modified to study the distribution of DNA methylation on metaphase chromosomes.